U.S. patent application number 12/489916 was filed with the patent office on 2010-12-23 for minimal access occipital plate.
This patent application is currently assigned to Aesculap Implant Systems, Inc.. Invention is credited to Ron Apfelbaum, Naveed Cheema, Fred Geisler, Larry Khoo.
Application Number | 20100324557 12/489916 |
Document ID | / |
Family ID | 43354949 |
Filed Date | 2010-12-23 |
United States Patent
Application |
20100324557 |
Kind Code |
A1 |
Cheema; Naveed ; et
al. |
December 23, 2010 |
MINIMAL ACCESS OCCIPITAL PLATE
Abstract
A bone plate for securing a spinal fixation element to bone
includes a base portion having a top face, a rod receiving portion
and a bone anchor receiving portion. The bone anchor receiving
portion includes a plurality of angled holes for receiving bone
screws. At least one of the screw holes is oriented at an acute
angle relative to the top face of the plate.
Inventors: |
Cheema; Naveed;
(Breinigsville, PA) ; Apfelbaum; Ron; (Salt Lake
City, UT) ; Geisler; Fred; (Aurora, IL) ;
Khoo; Larry; (Studio City, CA) |
Correspondence
Address: |
RATNERPRESTIA
P.O. BOX 980
VALLEY FORGE
PA
19482
US
|
Assignee: |
Aesculap Implant Systems,
Inc.
Center Valley
PA
|
Family ID: |
43354949 |
Appl. No.: |
12/489916 |
Filed: |
June 23, 2009 |
Current U.S.
Class: |
606/70 ;
606/286 |
Current CPC
Class: |
A61B 17/7055
20130101 |
Class at
Publication: |
606/70 ;
606/286 |
International
Class: |
A61B 17/80 20060101
A61B017/80 |
Claims
1. A bone plate for securing a spinal fixation element to bone, the
bone plate comprising: a base portion having a top face; a rod
receiving portion extending from the base portion, the rod
receiving portion including a channel for receiving a spinal
fixation element; and a bone anchor receiving portion comprising a
plurality of raised projections, each raised projection projecting
from the top face of the plate and forming a hole for receiving a
bone anchor into the raised projection and through the base
portion, each hole having a hole axis extending at an acute angle
with respect to the top face.
2. The bone plate of claim 1, wherein the axes of all of the holes
converge at a single point above the top face of the plate.
3. The bone plate of claim 1, wherein the bone anchor receiving
portion is characterized by an X-Y plane, a Y-Z plane and an X-Z
plane, and at least one hole axis is oriented at an acute angle
with respect to the top face in a plane parallel to one of the X-Y
plane, Y-Z plane and X-Z plane.
4. The bone plate of claim 3, wherein the holes comprise a superior
hole, an inferior hole, a first lateral hole and a second lateral
hole.
5. The bone plate of claim 4 wherein the axis of the superior hole
extends at an acute angle .THETA..sub.XZ relative to the top face,
and the inferior hole extends at an acute angle .THETA..sub.XZ
relative to the top face, wherein angle .THETA..sub.XZ for the
superior hole is greater than .THETA..sub.XZ for the inferior
hole.
6. The bone plate of claim 5, wherein angle .THETA..sub.XZ for the
superior hole is between about 55 degrees and about 65 degrees, and
angle .THETA..sub.XZ for the inferior hole is between about 47
degrees and about 57 degrees.
7. The bone plate of claim 4, wherein the axis of the first lateral
hole extends at an acute angle .THETA..sub.XZ relative to the top
face and an angle .THETA..sub.YZ relative to the top face, wherein
angle .THETA..sub.YZ for the first lateral hole is greater than
angle .THETA..sub.XZ for the first lateral hole.
8. The bone plate of claim 7, wherein the axis of the second
lateral hole extends at an acute angle .THETA..sub.XZ relative to
the top face and an angle .THETA..sub.YZ relative to the top face,
wherein angle .THETA..sub.YZ for the second lateral hole is greater
than angle .THETA..sub.XZ for the second lateral hole.
9. The bone plate of claim 8, wherein angles .THETA..sub.XZ for the
first and second lateral holes are between about 52 degrees and
about 62 degrees, and angles .THETA..sub.YZ for the first and
second lateral holes are between about between about 73 degrees and
about 83 degrees, the first and second lateral holes being
symmetrically arranged on the top face of the plate.
10. A bone plate for securing a spinal fixation element to bone,
the bone plate comprising: a base portion having a top face; a rod
receiving portion for receiving a spinal fixation element; and a
bone anchor receiving portion comprising a plurality of raised
projections, each raised projection projecting from the top face of
the plate and forming a hole for receiving a bone anchor into the
raised projection and through the base portion, each hole having a
hole axis extending at an acute angle with respect to the top face,
the axes of the holes converging toward one another as they extend
away from the top face of the plate.
11. The bone plate of claim 10, wherein the axes of all of the
holes converge at a single point above the top face of the
plate.
12. The bone plate of claim 10, wherein the bone anchor receiving
portion is characterized by an X-Y plane, a Y-Z plane and an X-Z
plane, and at least one hole axis is oriented at an acute angle
with respect to the top face in a plane parallel to one of the X-Y
plane, Y-Z plane and X-Z plane.
13. The bone plate of claim 10, wherein the holes comprise a
superior hole, an inferior hole, a first lateral hole and a second
lateral hole.
14. The bone plate of claim 13 wherein the axis of the superior
hole extends at an acute angle .THETA..sub.XZ relative to the top
face, and the inferior hole extends at an acute angle
.THETA..sub.XZ relative to the top face, wherein angle
.THETA..sub.XZ for the superior hole is greater than .THETA..sub.XZ
for the inferior hole.
15. The bone plate of claim 14, wherein angle .THETA..sub.XZ for
the superior hole is between about 55 degrees and about 65 degrees,
and angle .THETA..sub.XZ for the inferior hole is between about 47
degrees and about 57 degrees.
16. The bone plate of claim 13, wherein the axis of the first
lateral hole extends at an acute angle .THETA..sub.XZ relative to
the top face and an angle .THETA..sub.YZ relative to the top face,
wherein angle .THETA..sub.YZ for the first lateral hole is greater
than angle .THETA..sub.XZ for the first lateral hole.
17. The bone plate of claim 16, wherein the axis of the second
lateral hole extends at an acute angle .THETA..sub.XZ relative to
the top face and an angle .THETA..sub.YZ relative to the top face,
wherein angle .THETA..sub.YZ for the second lateral hole is greater
than angle .THETA..sub.XZ for the second lateral hole.
18. The bone plate of claim 17, wherein angles .THETA..sub.XZ for
the first and second lateral holes are between about 52 degrees and
about 62 degrees, and angles .THETA..sub.YZ for the first and
second lateral holes are between about between about 73 degrees and
about 83 degrees.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to apparatuses and
methods for treating the cervical spine, and more particularly to
an occipital bone plate that can be used when access and work space
around the occiput bone are limited.
BACKGROUND OF THE INVENTION
[0002] Occipitocervical fixation may be accomplished by using a
bone plate attached to the occiput bone on the posterior of the
skull, in conjunction with spinal rods. In many cases, the bone
plate is attached to the skull with bone screws. Drilling and
tapping holes at the rear of the skull is a difficult procedure
that requires a significant amount of force to penetrate the dense
cortical bone. The process is further complicated by the angle of
approach that is required for instruments. Because the plate is
positioned in close proximity to the back of the neck, the working
area around the plate location is extremely confined. Moreover, the
orientation of the plate is such that the plate extends more or
less normal to the axis of the cervical spine. Typical bone plates
feature holes with axes that extend normally to the surface of the
plate. In this arrangement, the axes of the screw holes are very
close to the axis of the cervical spine. Consequently, the surgeon
must position screw drivers, drills and taps right up against the
cervical spine to maintain the proper trajectory while drilling and
tapping the occiput bone, and driving bone screws through the
plate.
[0003] Conventional screw drivers, drills and taps have relatively
long shafts. To position these instruments so as to achieve the
proper trajectories, the shafts must be positioned normal to the
plate, right up against the cervical spine. Therefore, the surgeon
must make a long incision along the axis of the cervical spine to
make room to maneuver the instrument shafts. Large incisions are
clearly undesirable because they are invasive, introduce greater
risk of complications, and require long recovery times. Moreover,
longer incisions are not always effective in providing sufficient
clearance for instruments. Cervical deformities that obstruct the
area around the occiput bone, for example, can make it impossible
to position instrument shafts at the proper angle necessary to
access the screws.
[0004] For the foregoing reasons, conventional occipitocervical
fixation implants and techniques have many unresolved
drawbacks.
SUMMARY OF THE INVENTION
[0005] The drawbacks of conventional occipitocervical fixation
implants and techniques are resolved in many respects by bone
plates and assemblies in accordance with the present invention. In
a first aspect of the invention, a bone plate for securing a spinal
fixation element to bone includes a base portion having a top face,
a rod receiving portion and a bone anchor receiving portion. The
rod receiving portion extends from the base portion, and includes a
channel for receiving a spinal fixation element. The bone anchor
receiving portion includes one or more raised projections
projecting from the top face of the plate. Each projection forms a
hole for receiving a bone anchor into the raised projection and
through the base portion. Each hole has a hole axis extending at an
acute angle with respect to the top face.
[0006] In a second aspect of the invention, a bone plate for
securing a spinal fixation element to bone includes a base portion
having a top face, a rod receiving portion for receiving a spinal
fixation element, and a bone anchor receiving portion. The bone
anchor receiving portion includes one or more angled screw holes.
Each screw hole has a hole axis extending at an acute angle with
respect to the top face. The axes of two or more holes converge
toward one another as they extend away from the top face of the
plate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The foregoing summary and the following detailed description
will be better understood in conjunction with the drawing figures,
of which:
[0008] FIG. 1 is a perspective view of an occipitocervical fixation
assembly in accordance with the invention, schematically shown as
it would be positioned to stabilize a patient's cervical spine;
[0009] FIG. 2 is a front view of an exemplary occiput bone plate
with a pair of receiver elements for receiving spinal fixation
elements;
[0010] FIG. 3 is a side view of the occiput bone plate of FIG. 2,
with the rod receiver elements removed;
[0011] FIG. 4 is a side view of the occiput bone plate of FIG. 2,
shown in a schematic illustration of a technique for attaching the
plate to the occiput bone of a patient's skull in accordance with
the invention;
[0012] FIG. 5 is a top view of the occiput bone plate of FIG.
2;
[0013] FIG. 6 is a front view of the occiput bone plate of FIG. 2,
shown being engaged by an instrument in four possible
positions;
[0014] FIG. 7 is a side view of the occiput bone plate of FIG. 2,
shown with a plurality of bone screws inserted through the
plate.
[0015] FIG. 8 is a perspective view of a bone plate in accordance
with an alternative embodiment of the invention;
[0016] FIG. 9 is another perspective view of the bone plate of FIG.
8; and
[0017] FIG. 10 is a cross-sectional view of a skull illustrating
different screw trajectories through the occiput bone.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0018] Although the invention is illustrated and described herein
with reference to specific embodiments, the invention is not
intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims and without departing from the
invention.
[0019] Occipital bone plates in accordance with the present
invention provide a mechanism for stabilizing the base of the skull
and cervical spine, while allowing surgical instrumentation to be
held away from the cervical spine. In addition, bone plates in
accordance with the invention provide a mechanism that reduces the
size of incisions. Moreover, bone plates in accordance with the
invention provide a stronger engagement between the occiput bone
and bone screws. These combined benefits are achieved by selective
angulation of screw holes in the bone plate. By angulating screw
holes with respect to a reference plane, such as the plate's base
portion, the screw holes can be tapped and drilled, and bone screws
can be inserted and driven into the proper depth in the bone, with
sufficient work space and clearance. Bone plates in accordance with
the invention can be used with conventional drills, taps and other
instruments with elongated shafts. This avoids the need for special
instruments, like drills with flexible shafts, that are shaped and
designed to apply force from difficult approach angles. Flexible
drills are difficult to use, because the surgeon cannot easily
apply force behind the flexible drill shaft in the drilling
direction to penetrate the dense cortical bone.
[0020] In preferred embodiments, the bone plate generally includes
a base portion having a top face, a rod receiving portion and a
bone anchor receiving portion. The rod receiving portion connects
to one or more spinal fixation elements, such as spinal rods
implanted over the cervical spine. The bone anchor receiving
portion includes a number of holes or apertures designed to receive
and securely hold bone screws. The holes are formed in raised
projections that project from the top face of the base portion, and
each hole has a hole axis that controls the orientation of the bone
screw to be received in that hole.
[0021] Each raised projection is designed to orient the hole axis
of the corresponding screw hole in a predetermined angle. The
angular orientation of each hole axis allows the surgeon to
position instruments away from the patient's spine. More
specifically, the angular orientation of the holes allow elongated
shafts of drills, taps and drivers to access the plate at acute
angles relative to the axis of the cervical spine, thereby
positioning the instrument's body out and away from the patient's
spine. As explained in more detail below, preferred embodiments of
the invention have hole angulations that collectively minimize the
incision through which instruments access the plate. This can be
achieved by orienting two or more hole axes in a converging
arrangement, so that the axes intersect at a point above the
plate.
[0022] The angular orientation of the screw hole axes may be
measured with respect to any reference plane, such as an imaginary
plane normal to the occipital bone at a point where the plate
contacts the bone. For example, using the perimeter of the plate
section that contacts the bone, the centroid of the plate's
footprint contacting the bone may be selected as the point through
which to define the reference plane, which may be tangential or
parallel to the bone surface at that point. Alternatively, the
reference plane may be a plane that passes through the top face of
the plate, assuming the top face is flat or has flat sections. The
top face of the plate need not be flat, however, as various
curvatures and contours may be incorporated on the plate's top
surface in accordance with the invention. Where the top face is
curved, a reference plane that is tangential to a point on the top
face may be used to define the angular orientation of a hole axis.
Accordingly, angular orientations of screw hole axes may be
measured based on various reference planes that include but are not
limited to planes that coincide with or are tangential to the top
face of the plate, or planes that coincide with or are tangential
to a section of occipital bone.
[0023] Referring now to FIG. 1, a fixation system 20 is shown in
accordance with one exemplary embodiment of the invention,
illustrated schematically with portions of a skull S and cervical
vertebrae CV. Fixation system 20 includes a pair of spinal fixation
rods 30 that are attached to cervical vertebrae in the spine. Each
rod 30 has a curved or bent end 32 that is secured to the base of
the skull, thereby creating a fixation member that stabilizes the
cervical vertebrae and skull in a fixed position. Rod ends 32 are
secured to the base of the skull by a bone plate 100 that is
attached to the occiput bone OB of the skull.
[0024] Various directional terms are used herein to describe
relative positions and directions. Unless otherwise specified, the
term "posterior" refers to a position or direction toward the
patient's back side. Moreover, unless otherwise specified, the term
"superior" refers to features that are positioned toward the
patient's skull after implantation, and the term "inferior" refers
to features that are positioned toward the patient's feet after
implantation, relative to a corresponding superior feature.
Furthermore, unless otherwise specified, the term "lateral" refers
to features that would be positioned toward the patient's left side
or right side after implantation, relative to the patient's
spine.
[0025] Referring now to FIG. 2, bone plate 100 includes a base
portion 110 having a flat top face 112, a superior end 114 and an
inferior end 116. A rod receiving portion 120 extends from inferior
end 116. Rod receiving portion 120 includes a pair of arms 122,
with each arm having a front face 123 and a generally cylindrical
receiver body 124 extending from the front face. Each receiver body
124 includes a U-shaped channel 126 adapted to receive one of the
rod ends 32. Each rod end 32 bends in a posterior direction into
alignment with one of the U-shaped channels 126 on the receiver
bodies 124 on bone plate 100. Rod ends 32 can be secured in
receiver bodies 124 by any type of fastener, such as set screws
having external threads that engage threading in the U-shaped
channels 126.
[0026] Bone plates in accordance with the invention include a bone
anchor receiving portion designed to receive bone screws at
selected angles, as noted above. The selected angles allow
instruments to be tilted away from the cervical spine, so that the
surgeon only needs to position a small part of the shaft's distal
end near the cervical spine. Tilting or angling the instrument away
from the axis of the cervical spine provides more working room for
the surgeon.
[0027] In preferred embodiments, the hole angles not only to allow
tilting of instrument shafts away from the spine, but also
cooperate with one another to minimize the required size of the
incision. For example, screw holes may be angled so that their axes
converge toward one another and intersect on or near a point of
incision on the patient's skin. In this arrangement, the shaft of a
drill (or other instrument) can be inserted through one small
incision and pivoted within that incision to access each screw
hole. The surgeon is able to direct the tip of the instrument shaft
to each screw hole through the same small incision, without the
need to make one large incision or multiple small incisions to
access each screw hole location. It is preferable that at least two
of the screw hole axes converge toward one another, and more
preferable that all the screw hole axes converge toward one another
so as to intersect at one or more points above the plate location.
Even more preferably, all of the screw holes in the plate converge
toward one another and intersect at a single point which, after the
plate is implanted, coincides with a single point of incision on
the patient's skin.
[0028] The screw hole axes may be angled in a number of ways to
guide the trajectory of instruments and bone screws. For example,
the screw hole angles may be provided by oblique passages that
extend through the plate at desired angles. Alternatively, the
screw hole angles can be provided by oblique passages in
combination with raised protruberances, such as raised projections
or bosses, that extend out from the top face of the plate. The
protuberances provide a more convenient way to manufacture the
angled holes. Referring to FIG. 2, for example, bone plate 100
includes a bone anchor receiving portion 130 with four raised
projections or bosses 132. Projections 132, which are formed when
the plate is stamped, are rounded hubs with relatively flat end
faces 134. Each end face 134 has a screw hole 140 for receiving a
bone screw or other anchoring element through the plate.
[0029] Screw holes 140 may be formed through end faces 134 by
punching the holes through the end faces. Each end face 134 is
planar, or more or less follows a plane. The plane of each end face
preferably extends normal to the screw hole axis that passes
through end face. Screw holes in accordance with the invention may
include a variety of seat configurations for receiving the heads of
bone screws. In FIG. 2, screw holes 140 contain conical shaped seat
surfaces 142 for receiving monoaxial screws.
[0030] Bone plates in accordance with the invention may have a
number of different hole sizes, configurations and arrangements. As
discussed above, and as will be explained in more detail below, the
hole sizes, hole configurations and hole arrangements can be
selected in different combinations to achieve a number of benefits.
One hole arrangement in accordance with the invention, shown in
FIG. 2, features four screw holes 140 arranged in a diamond-shaped
configuration. Holes 140 include a superior hole 140a, an inferior
hole 140b, a first lateral hole 140c and a second lateral hole
140d. Plates in accordance with the invention may have fewer holes
or more holes, and need not be limited to four.
[0031] Holes 140a-140d have longitudinal axes 141a-141d,
respectively, that extend through the plate and define the final
orientations of the screws that are inserted through the screw
holes. Referring now to FIGS. 2, 3 and 5, hole axes 141a-141d form
acute angles with top face 112 of plate 100 and converge toward one
another as they extend away from the top face as shown. For
purposes herein, the orientations of hole axes 141a-141d are
described in terms of angles based on three reference axes, X, Y
and Z shown in the drawings, and imaginary planes between the axes.
The Figures show three planes of reference, which include the X-Y
plane coinciding with top face 112 of plate 100 (see FIG. 2), the
Y-Z plane that extends normal to top face 112 (see FIG. 5), and the
X-Z plane that extends perpendicularly to the X-Y and Y-Z planes
(FIG. 3).
[0032] Each hole axis is characterized by an angle .THETA..sub.XZ
relative to top face 112 that extends in a plane parallel to the
X-Z plane. Examples of these angles are shown in FIG. 3. Each hole
axis also has an angle .THETA..sub.YZ, which is the angle relative
to top face 112 extending in a plane parallel to the Y-Z plane.
Examples of these angles are shown in FIG. 5.
[0033] Referring now to FIG. 3, a side view of bone plate 100 is
shown. From this side view, it is apparent that all four
projections 132 are oriented with their end faces and screw holes
angled toward superior end 114 of bone plate 100. That is, the axes
of screw holes 140a-140d extend at acute angles .THETA..sub.XZ with
respect to top face 112. The axis of the superior screw hole 140a
preferably forms an angle .THETA..sub.XZ of between about 55 and
about 65 degrees, more preferably between about 58 degrees and
about 62 degrees, and even more preferably at an angle of about 60
degrees. The axes of the first and second lateral holes 140c and
140d preferably form an angle .THETA..sub.XZ of between about 52
and about 62 degrees, more preferably between about 55 degrees and
about 59 degrees, and even more preferably at an angle of about 57
degrees. Lastly, the axis of inferior screw hole 140b forms an
angle .THETA..sub.XZ of between about 47 degrees and about 57
degrees, more preferably between about 50 degrees and about 54
degrees, and even more preferably at an angle of about 52
degrees.
[0034] The hole orientations described above result in screw holes
that are angled upwardly or toward superior end 114 of plate 100 as
the holes extend toward the top face. By angling screw holes toward
the superior end of a bone plate, instruments can engage the plate
easily using an "angled approach." FIG. 4 schematically illustrates
a benefit of using an angled approach provided by the invention. In
FIG. 4, bone plate 100 is in the process of being secured to a
patient's occiput bone OB. The surface of the patient's skin is
schematically illustrated by the skin line "SL". A screw driver D
approaches a screw hole in plate 100 at a relatively small angle
with respect to the plate, to drive a bone screw through the plate.
The orientations of projections 132 and the screw hole axes allow
screw driver D to approach the plate from a position that is offset
away from the spine (i.e. posteriorly to the cervical spine axis).
As such, screw driver D is advanced toward the screw holes from a
position that is mostly above skin line SL. Only a very small
section of screw driver D at the screw driver's distal end must be
positioned near skin line SL. Accordingly, plate 100 only requires
a very small incision in proximity to the plate to provide access
for long screw drivers and other instrumentation.
[0035] Plates that feature hole axes oriented normally to the
mounting surface of the occiput bone surface do not permit
conventional bone screws and long shafted instruments from
approaching the plate in an angled approach. Instead, the shafts of
instruments must be held normal to the plate, or substantially
normal to the plate, as represented by line "H" in FIG. 4. In such
a position, there is little or no clearance around the shaft,
making it difficult to manipulate the instrument and advance bone
screws at the proper trajectories into the plate holes. In some
cases, the entire shaft of screw driver D must be positioned inside
the patient beneath skin line SL, more or less adjacent to the axis
of the cervical spine. To provide sufficient space around the
shaft, a long incision must be made above the cervical spine to
receive the entire length of screw driver D and allow the screw
driver end to approach the screw holes from a direction normal to
plate P. Large incisions are undesirable, as noted above, because
they are very invasive, increase risks of complications during and
after surgery, and lengthen recovery time, as compared to
procedures using smaller incisions. Moreover, placing an instrument
inside an incision above the cervical spine is not always possible.
Spinal deformities and other factors can create obstructions that
prevent a long instrument from being placed in the incision and
advanced in a direction normal to the plate.
[0036] Another advantage of angling the screw holes toward the
superior end of bone plate 100 is improved contact between the bone
screws and the occiput bone. The occiput bone is a saucer shaped
bone which varies in thickness along its length. FIG. 10 shows a
cross-section of a skull, showing the thickness variation along the
length of occiput bone OB. A bone plate BP is attached to occiput
bone OB, and two possible screw trajectories T.sub.1 and T.sub.2
are shown. Screws that are advanced through plate BP along an axis
normal to the plate would follow trajectory T.sub.1. Screws that
are advanced through bone plate BP at the same point, but using an
angled approach in accordance with the invention, would follow
trajectory T.sub.2. In trajectory T.sub.2, the screw shanks extend
through a longer length of bone than in trajectory T.sub.1. That
is, the orientation of trajectory T.sub.2 traverses a greater
length of bone than trajectory T.sub.1. Therefore, trajectory
T.sub.2 provides a greater degree of bone penetration than
trajectory T.sub.1, allowing for greater screw purchase in the
bone, and accommodating longer bone screws, if desired.
[0037] Preferred plates in accordance with the invention provide
hole axes that are angled toward the superior end of the plate, as
noted above. The holes axes may be parallel to one another. In more
preferred embodiments, however, the holes axes converge toward one
another as they extend away from the top plate, and consequently
diverge away from one another as they extend into the bone.
Referring to FIG. 5, a top view of bone plate 100 is shown, looking
into superior end 114 of the plate. Projections 132 associated with
first and second lateral holes 140c and 140d are oriented with
their end faces toward the center of bone plate 100. The axes of
screw holes 140c and 140d extend at acute angles .THETA..sub.YZ
with respect to top face 112. In contrast, the axes of superior
screw hole 140a and inferior screw hole 140b are not angled in the
Y-Z direction, with their respective angles .THETA..sub.YZ being
right angles. The axes of first and second lateral holes 140c and
140d preferably extend at angles .THETA..sub.YZ of between about 73
and about 83 degrees, more preferably between about 76 degrees and
about 80 degrees, and even more preferably at an angle of about 78
degrees.
[0038] Based on the foregoing hole angles, hole axes 141a-141d
converge toward one another as they extend outwardly from top face
of plate 112 (i.e. they converge as they extend in the posterior
direction away from the skull when the plate is implanted), and
diverge from one another as they extend in the opposite direction.
This has multiple benefits. By converging in the posterior
direction, the axes allow the surgeon to minimize the size of the
incision needed to access the plate. The intersecting axes allow
instruments to access the plate through very small incisions, or
even a single incision, depending on where they intersect. In a
preferred embodiment, hole axes 141a-141d are oriented so as to
converge at a single point on the patient's skin, which corresponds
to a single entry point for an instrument. This configuration
allows an instrument to access all of the screw holes while
extending from a single entry point through the patient's skin, as
illustrated schematically in FIG. 6. FIG. 6 shows screw driver D
inserted through a single entry point P of a patient's skin S, in
four possible positions (with three positions shown in phantom
lines for clarity). Screw driver D accesses a different screw hole
in each of the illustrated positions. The screw hole axes intersect
at point P. Consequently, the shaft of the screw driver intersects
point P in each of the four positions. As such, a single small
incision at entry point P will accommodate the screw driver D and
allow it to access each of the four screw holes.
[0039] Hole axes 141a-141d diverge away from one another as they
extend in the bone, as noted above. This has the additional benefit
of improving the anchorage of the plate. FIG. 7 shows one possible
arrangement in which screws are inserted through screw holes
140a-140d. The diverging hole axes allow the shanks of the bone
screws to spread apart from one another in the bone, providing a
wider support structure. Because the screw axes are non-parallel,
the plate is reinforced against pull-out in multiple
directions.
[0040] Referring now to FIGS. 8 and 9, a bone plate 200 is shown in
accordance with an alternative embodiment of the invention. Bone
plate 200 is similar in many respects to bone plate 100 but
features a modified rod receiving portion 220. Plate 200 includes a
superior end 214, inferior end 216, flat top face 212 and four
angled screw holes 240 that project outwardly from the top face.
Rod receiving portion 220 includes a pair of arms 222 with front
faces 223 that are tilted upwardly toward the superior end 214 of
plate 200. This is in contrast to arms 122 on plate 100, which have
faces 123 oriented more or less parallel to top face 112, as seen
in FIG. 3. The tilted faces 223 on plate 200 allow rod receivers to
be mounted on arms 222 so that the openings in the rod receivers
are angled upwardly, similar to screw holes 240. FIGS. 8 and 9 are
shown without rod receivers on arms 222 so that the entire faces
223 can be seen. When plate 200 is attached to the skull, the
tilted arms 222 support rod receivers with their openings angled
upwardly so that instruments can access the rod receivers with the
same angled approach used to access the screw holes.
[0041] While preferred embodiments of the invention have been shown
and described herein, it will be understood that such embodiments
are provided by way of example only. Numerous variations, changes
and substitutions will occur to those skilled in the art without
departing from the spirit of the invention. Accordingly, it is
intended that the appended claims cover all such variations as fall
within the spirit and scope of the invention.
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